Monday, 29 August 2011

3.24 Mitosis

Understand that division of a diploid cell by mitosis (cell division) produces two cells which contain identical sets of chromosomes.
Cell division => growth/increase in number of cells

a.
The Diploid number is the number of chromosomes in the nucleus.
This is sometimes abbreviated to the symbol 2n, for humans the number is 46. So, 2n=46 for humans, for cats 2n=38. In the process known as mitosis a cell is divided to two cells - each with a nucleus. If we look at the nucleus we will find that each one will have a diploid nucleus. We often say these cells are identical or we can call it as daughter cells. (they have the same number of chromosomes and they have the same set of chromosomes.)

b. How are copies made and How do they separate?
Mitosis is done through a process called DNA replication, in this process each chromosome under goes a copying process to form another identical copy of itself of the same genes and alleles. The two copies are held together by a structure around the center region known as the centromere. We refer to them as a pair of chromatids. The process of DNA replication takes place inside the nucleus while its still inact. This is known as the interphase of the cell cycle.

C. The stages of mitosis

Observing a cell through a microscope, we will see a nucleus has a spherical structur and would be unable to see the chromosomes. It is during the interphase that DNA replication occurs.

Prophase is where the breaking down of a membrane occurs and the chromosomes become visible, and they are visible as a pair of chromotids.

The nucleus will be gone after prophase, leaving the cell with protein molecules called spindle. These extend from one pole of a cell to another. The pair of chromatids will soon join on one of the spindle fibres at the centromere.Metaphase is when the pair of chromatids are attached to a spindle fibre by the centromere, characteristic of the metaphase is that the chromatid is in the middle across the equator of the cell.In the Anaphase the spindle fibre shortens pulling chromotids into different direction (apart) and they move to the poles of the cell and seperates.Telophase is the last phase of mitosis, in which the nucleus reforms around the chromosomes at either ends. We see the formation of two nuclei at opposite ends of the cell.Cytokinesis is the process in which the cell splits into two which is not part of mitosis. Here we see the cytoplasm of the cell split and eventually forms two cells.